Process for electrolytic polishing of aluminum



United States Patent 3 Claims. 01. 204-1405 This invention relates to aprocess for electrolytic polishing, and more particularly it relates toa process for developing pear grained aluminum surface. Anelectropolishing agent of the present invention characteristicallycontains alkali gluconate and alkali hydroxide.

The term pear appearance used in the specification means that thesurface is similar to a pear skin or .uneven, and it is to be construedin somewhat broader sense, including coarsely uneven and finely unevensurfaces with or without luster in both cases.

In the prior art for finishing aluminum surface in pear appearance, amethod of spraying sand, Alundum, Carborundum or other particles ontothe surface and a method of brushing the surface with a rotary wirebrush are widely employed. These prior methods, however, aredisadvantageous because they usually stain the aluminum surface to bepolished.

As the result of our studies it has now been found that whereelectrolysis is carried out by using an electrolytic solution containingmore than 1.7 molal concentration of alkali hydroxide in water andalkali gluconate and aluminum anode, good pear appearance can bedeveloped on the surface of aluminum. Preferred as alkali hydroxide issodium hydroxide or potassium hydroxide. In some cases, a part or thewhole of the alkali hydroxide may be substituted with barium hydroxide,sodium carbonate or potassium carbonate, although the optimum resultwill be obtained by the use of sodium or potassium hydroxide. Alkalihydroxide 'is suitable at any molal concentration of more than 1.7moles/ 1000 g. of Water. Although more than moles of alkali hydroxidemakes the solution viscous, alkali hydroxide saturated solution may beused if a highly viscous solution state is permissible. Where the alkalihydroxide concentration is less than 1.5 M., the aluminum anode surfacetends to be developed smoothly rather than having a pear grainedsurface. Preferred alkali gluconates are sodium gluconate, potassiumgluconate, calcium gluconate or the like. Among these, sodium andpotassium gluconates 'ice pared with calcium gluconate. Particularlysodium gluconate is recommendable because of its easy availability. Ifalkali gluconate is absent in the electrolyte, the aluminum surface isnot developed to pear appearance but becomes a lusterless white color.The higher the gluconate concentration, the more a lustrous uniform pearappearance can be. obtained. Suitable gluconate concentration may rangefrom 0.1 M. to saturation. Pear appearance can be developed more or lessat a relatively low gluconate concentration, if the concentra, tion iswithin the above-indicated critical range. An electropolishing solutionof the present invention may, of course, contain other ingredients, suchas .clrromates, non-ionic surface active agents, etc., withoutpreventing the development of pear appearance on aluminum surface. Theelectrolyzing temperature can be within the range of 0 to 100 C. Fromthe technical viewpoint, however, a temperature of from 15 to 60 C. ispreferred because heating or cooling means; are made unnecessary. Theuseof a high temperature facilitates to increase electroconductivity ofthe solution used, and under this circumstance it is possible toincrease an electric current density at a low voltage. The currentdensity may vary depending upon the composition of the solution,temperature and other conditions for electrolysis. Usually it is foundthat the electric current density is maximum at the starting period ofelectrolysis and then gradually lowers to show an approximately constantvalue to 120 seconds after initiation of the electrolysis. Over therange of 3 to 100 amperes per square decimeter as constant value thealuminum surface can be polished to pear appearance. Practically, anelectric current density of 54() amperes per square decimeter issuitable. The time required for electrolysis is 0.5 to 30 minutes andpractically 1.5 to 2.5 minutes is sufiicient.

The aluminum may be either pure or somewhat impure. However, aluminumbeing more than 98% pure is particularly suitable. Pure aluminum can bepolished to a little more lustrous pear appearance as compared withimpure aluminum, but there is no substantial difference in otherfeatures. A suitable cathode for electropolishing according to thepresent invention is aluminum, iron or carbon electrode. In order tocarry out conventional electropolishing it is necessary to bolt anddegrease the aluminum surface prior to the electropolishing. However, inthe present invention, the aluminum surface can be successfully polishedto pear appearance directly without bufiing and degreasing. Scars on thealuminum surare preferable because of their high solubility when com- 50face can be polished or, even 1f they remain, are not Electrolyte Finalcurrent Temp. density Time Run No. Sodium Hexaoxy- Sodium C.) (A/clmfi)1 (min) Remark gluconate Caustic ethylene chromate Water (kg) (mol) soda(mol) laurylether (mol) (mol) 0. 5 2 1 30 15 2 Semi-lustrous, coarsepear ap pearenee.

1 3 1 30 15 2 Lustrous, somewhat coarse pear appearence.

2 3 1 30 15 2 Lustrous pear appearence.

1 Amperes per square decimeter.

noticeable because they usually are overlapped with roughness of pearappearance. However, if several scars, are present, bufiing may berequired. If the aluminum surface is stained with a considerable amountof grease, it should be preferably degreased. Bufiing and degreasingtreatments can be effected by simple procedures which are known.

The examples showing the nature of the present invention in details aresummarized in the above table.

In all these runs, 99.7%-pure aluminum plate was used. Similar resultscan be obtained when 0.5 M. sodium chromate and/or 0.03 M. nonionicsurface active agents are added to the electrolytic solution used in anyof these runs. Runs No. 1 and 2 which were entirely free of gluconateare control experiments showing the effect of the present invention. InRuns No. 5 to No. 9 which employed more than 2 M. gluconate and morethan 3 M. caustic soda, lustrous, beautiful pear appearance wasobtained. The aluminum having been polished to pear appearance can besuccessfully stained by subjecting same to an anodic oxidation in asulfuric acid solution and then immersing in a dye solution.

What I claim is:

- 1. Process for effecting pear-like appearance on aluminum whichcomprises immersing aluminum in an electropolishing aqueous solutionconsisting essentially of water, at least 0.1 mol per kilogram of waterof alkali metal gluconate and at least 1.7 mol per kilogram of water ofalkali metal hydroxide, and electrolyzing, wherein the aluminum is theanode, at a current density of 3 to 100 amperes per square decimeteruntil pear-like appearance on said aluminum is attained.

2. Process of claim 1 wherein the electrolysis is effected for 0.5 tominutes.

3. Process of claim 1 wherein said current density is 5 to 20 amperesper square decirneter and said electrolysis is effected for 1.5 to 2.5minutes.

References Cited by the Examiner UNITED STATES PATENTS 2,550,544 4/1951Faust 204-140.5 2,967,136 1/ 1961 Cybriwsky 204140.5

FOREIGN PATENTS 510,138 2/1955 Canada. 513,530 3/1939 Great Britain.521,290 5/ 1940 Great Britain.

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner.

1. PROCESS FOR EFFECTING "PEAR-LIKE" APPEARANCE ON ALUMINUM WHICHCOMPRISES IMMERSING ALUMINUM IN AN ELECTROPOLISHING AQUEOUS SOLUTIONCONSISTING ESSENTIALLY OF WATER, AT LEAST 0.1 MOL PER KILOGRAM OF WATEROF ALKALI METAL GLUCONATE AND AT LEAST 1.7 MOL PER KILOGRAM OF WATER OFALKALI METAL HYDROXIDE, AND ELECTROLYZING, WHEREIN THE ALUMINUM IS THEANODE, AT A CURRENT DENSITY OF 3 TO 100 AMPERES PER SQUARE DECIMETERUNTIL "PEAR-LIKE" APPEARANCE ON SAID ALUMINUM IS ATTAINED.